decouple parser and EvalState

there's no reason the parser itself should be doing semantic analysis
like bindVars. split this bit apart (retaining the previous name in
EvalState) and have the parser really do *only* parsing, decoupled from
EvalState.

src/libexpr/eval.cc

@@ -22,6 +22,7 @@
 #include "fetch-to-store.hh"
 #include "tarball.hh"
 #include "flake/flakeref.hh"
+#include "parser-tab.hh"
 
 #include <algorithm>
 #include <chrono>
@@ -2800,6 +2801,21 @@ std::optional<std::string> EvalState::resolveSearchPathPath(const SearchPath::Pa
 }
 
 
+Expr * EvalState::parse(
+    char * text,
+    size_t length,
+    Pos::Origin origin,
+    const SourcePath & basePath,
+    std::shared_ptr<StaticEnv> & staticEnv)
+{
+    auto result = parseExprFromBuf(text, length, origin, basePath, symbols, positions, rootFS);
+
+    result->bindVars(*this, staticEnv);
+
+    return result;
+}
+
+
 std::string ExternalValueBase::coerceToString(const Pos & pos, NixStringContext & context, bool copyMore, bool copyToStore) const
 {
     throw TypeError({